Nebulizers for producing aerosol for use in respiratory therapy are well known. A popular and efficient nebulizer incorporates a vertical air nozzle having a lower end into which gas is directed and an upper gas exit orifice, the air nozzle being surrounded by a concentric aspirator cap. A bowl forming a reservoir for holding water and/or medication communicates with channels extending between the nozzle and aspirator cap, the latter having a spray orifice at the upper end, also concentric with the gas orifice. An impingement target, preferably convex, is located directly above the spray orifice of the aspirator cap. Aerosol created by the device impinges against the target surface and then passes through an outlet pipe. Such a nebulizer is shown in U.S. Pat. No. 3,826,255 and another similar device in U.S. Pat. No. 3,762,409. Although such nebulizers produce an adequate aerosol, the particle size distribution is not entirely satisfactory. Particularly, in the latter device, a substantial percentage of particles passing through the outlet pipe have a particle size of 10 microns or greater. The present invention is directed to a device which generates an aerosol having a particle size of less than 10 microns, preferably a substantial amount of particles in the 1-5 micron range, and more preferably a majority of the particles in the 1-3 micron size interval.